CVE-2026-6739: Github.com/mattermost/mattermost/server/v8 Mattermost Doesn't Require System-level Permission Vulnerability

MODERATE
Advisory Identifier: GHSA-m2w9-h2mm-79qr
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CVSS 6.7
CWE-863
Published
Advisory Summary

CVE-2026-6739 is a MODERATE (CVSS 6.7) mattermost doesn't require system-level permission vulnerability (CWE-863: Incorrect Authorization) affecting Github.com/mattermost/mattermost/server/v8 libraries in the Go ecosystem. The issue involves mattermost doesn't require system-level permission when patching protected default system roles. Remediation is available by upgrading affected packages to version 8.0.0-20260501142004-99b73d4c4acf, 11.6.1, 11.5.5, 10.11.17.

CVE-2026-6739 at a Glance

CVE Identifier CVE-2026-6739
GHSA Identifier GHSA-m2w9-h2mm-79qr
Severity MODERATE
CVSS v3.1 Base Score 6.7
CWE Classification CWE-863: Incorrect Authorization
Affected Product Github.com/mattermost/mattermost/server/v8
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 6.7

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required High (PR:H)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity High (I:H)
Availability Low (A:L)

How to Fix CVE-2026-6739

To remediate this vulnerability, upgrade all affected Github.com/mattermost/mattermost/server/v8 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

github.com/mattermost/mattermost/server/v8 fixed: 8.0.0-20260501142004-99b73d4c4acf 
Upgrade github.com/mattermost/mattermost/server/v8 to 8.0.0-20260501142004-99b73d4c4acf
github.com/mattermost/mattermost-server fixed: 11.6.1 
Upgrade github.com/mattermost/mattermost-server to 11.6.1
github.com/mattermost/mattermost-server fixed: 11.5.5 
Upgrade github.com/mattermost/mattermost-server to 11.5.5
github.com/mattermost/mattermost-server fixed: 10.11.17 
Upgrade github.com/mattermost/mattermost-server to 10.11.17
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for github.com/mattermost/mattermost/server/v8

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-6739, inspect your project dependencies for Github.com/mattermost/mattermost/server/v8:

inspect dependencies for github.com/mattermost/mattermost/server/v8

Analyze the terminal output to locate any version of Github.com/mattermost/mattermost/server/v8 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 8.0.0-20260501142004-99b73d4c4acf), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-863: Incorrect Authorization, where the product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. In Github.com/mattermost/mattermost/server/v8, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to require system-level permission when patching protected default system roles, which allows authenticated users with delegated user-management permissions to escalate privileges by altering built-in role permissions via the role patch API. Mattermost Advisory ID: MMSA-2026-00656

CVE-2026-6739 Affected Github.com/mattermost/mattermost/server/v8 Versions

4 component(s)
Fixed Versions:
8.0.0-20260501142004-99b73d4c4acf11.6.111.5.510.11.17
Product / Package Ecosystem Affected Versions Fixed Version
github.com/mattermost/mattermost/server/v8
golang/github.com/mattermost/mattermost/server/v8
Go
≥ 8.0.0-20250731163400-5b955468ea1e
< 8.0.0-20260501142004-99b73d4c4acf
8.0.0-20260501142004-99b73d4c4acf
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.6.0
< 11.6.1
All 1 versions
11.6.0
11.6.1
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 11.5.0
< 11.5.5
11.5.5
github.com/mattermost/mattermost-server
golang/github.com/mattermost/mattermost-server
Go
≥ 10.11.0
< 10.11.17
10.11.17

References

NVD — CVE-2026-6739 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-6739
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36197
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36377
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36379
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36380
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost/pull/36382
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/2c89c2f6768fbe4dfd57a21ca38c0aecead8d4a8
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/5e159647b16e571b327ac6882f32eae42971f540
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/8000e5933526f4fd66b92131db3a1b1f4520dbae
Security Fix Commit FIX COMMIT
https://github.com/mattermost/mattermost/commit/f0a390b96e4c730daedbaf5190684776730218c7
Github.com/mattermost/mattermost/server/v8 Source Repository REPOSITORY
https://github.com/mattermost/mattermost
Github.com/mattermost/mattermost/server/v8 Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v10.11.16
Github.com/mattermost/mattermost/server/v8 Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.5.5
Github.com/mattermost/mattermost/server/v8 Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.6.2
Github.com/mattermost/mattermost/server/v8 Release RELEASE
https://github.com/mattermost/mattermost/releases/tag/v11.7.0
Github.com/mattermost/mattermost/server/v8 Security Information SECURITY
https://mattermost.com/security-updates

Frequently Asked Questions

What is CVE-2026-6739?
CVE-2026-6739 is a security vulnerability in Github.com/mattermost/mattermost/server/v8 involving Mattermost doesn't require system-level permission when patching protected default system roles under CWE-863: Incorrect Authorization.
Which Github.com/mattermost/mattermost/server/v8 versions are affected?
Affected versions include releases prior to 8.0.0-20260501142004-99b73d4c4acf, 11.6.1, 11.5.5, 10.11.17 across affected packages: github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
What version fixes CVE-2026-6739?
Upgrading to 8.0.0-20260501142004-99b73d4c4acf, 11.6.1, 11.5.5, 10.11.17 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/mattermost/mattermost/server/v8 affected?
Yes, github.com/mattermost/mattermost/server/v8 is affected in releases prior to 8.0.0-20260501142004-99b73d4c4acf. Upgrade your dependency to 8.0.0-20260501142004-99b73d4c4acf or later.
Is github.com/mattermost/mattermost-server affected?
Yes, github.com/mattermost/mattermost-server is affected in releases prior to 11.6.1, 11.5.5, 10.11.17. Upgrade your dependency to 11.6.1, 11.5.5, 10.11.17 or later.
What is the CVSS score of CVE-2026-6739?
CVE-2026-6739 has a CVSS v3.1 base score of 6.7 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L.
What CWE is associated with CVE-2026-6739?
CVE-2026-6739 is classified under CWE-863: Incorrect Authorization (The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.).
How can I check Go dependencies for CVE-2026-6739?
Run the dependency tree command: 'inspect dependencies for github.com/mattermost/mattermost/server/v8'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m2w9-h2mm-79qr",
  "modified" : "2026-08-24T21:43:02Z",
  "published" : "2026-06-12T18:31:59Z",
  "aliases" : [ "CVE-2026-6739" ],
  "summary" : "Mattermost doesn't require system-level permission when patching protected default system roles",
  "details" : "Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to require system-level permission when patching protected default system roles, which allows authenticated users with delegated user-management permissions to escalate privileges by altering built-in role permissions via the role patch API. Mattermost Advisory ID: MMSA-2026-00656",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.6.0"
      }, {
        "fixed" : "11.6.1"
      } ]
    } ],
    "versions" : [ "11.6.0" ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "11.5.0"
      }, {
        "fixed" : "11.5.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 11.5.4"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost-server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "10.11.0"
      }, {
        "fixed" : "10.11.17"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 10.11.15"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/mattermost/mattermost/server/v8"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.0.0-20250731163400-5b955468ea1e"
      }, {
        "fixed" : "8.0.0-20260501142004-99b73d4c4acf"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-6739"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36197"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36377"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36379"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36380"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/pull/36382"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/2c89c2f6768fbe4dfd57a21ca38c0aecead8d4a8"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/5e159647b16e571b327ac6882f32eae42971f540"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/8000e5933526f4fd66b92131db3a1b1f4520dbae"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/commit/f0a390b96e4c730daedbaf5190684776730218c7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mattermost/mattermost"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v10.11.16"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.5.5"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.6.2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mattermost/mattermost/releases/tag/v11.7.0"
  }, {
    "type" : "WEB",
    "url" : "https://mattermost.com/security-updates"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-863" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-24T21:41:52Z",
    "nvd_published_at" : "2026-06-12T17:16:27Z"
  }
}